Our brains cannot differentiate between good and bad habits, all it cares about is getting the dopamine spike, that comes with particular behaviours.

The dopamine reward system works in 3 steps.

The first is a Cue, this is the start of a habit,

The next step is the routine which is the action set in motion by the cue in order to get the stimulus which will lead to your last phase of a habit

That is the all important reward.

So.. It is Cue, Routine and Reward.

Take eating junk food for example.

Cue.. Feeling of hunger.

Routine..Going into the kitchen and eating a chocolate bar (Or 3).

Reward.. The feel good sensation of eating chocolate.

This is a very powerful system which makes it very difficult to resist. Our brains do it without thinking.

This is why getting off the couch and going for a run is so difficult.

You have no previous experience of getting out and going for a run, or how it feels to own that sense of accomplishment. You don't have a feel good reward for your brain to crave and no Cue or Routine set in place.

It is said that it is almost impossible to just ignore and to just stop doing any given habit but it must be modified and replaced and in order to start new habits you should build on top of old ones.

For example. If you want to start running it would be a good idea to incorporate it into your already ingrained morning/evening routine and build on top of that, or if you don't have any kind of routine in place then set up a cue such as placing your running shoes waiting at the door. When your eyes see the shoes, it acts as a trigger for your brain to begin to crave the dopamine reward response that accomplishing a run brings.